CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Sanofi Genzyme	Common Stock	Common Stock Sanofi Genzyme	Additional Paid-in Capital	Additional Paid-in Capital Sanofi Genzyme	Total Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit
Beginning Balance at Dec. 31, 2016	$ 920,221		$ 859		$ 2,609,614		$ (33,441)	$ (1,656,811)
Beginning Balance (in shares) at Dec. 31, 2016			85,941,000
Exercise of common stock options, net of tax withholdings	80,546		$ 19		80,527
Exercise of common stock options, net of tax withholdings (in shares)			1,842,000
Issuance of common stock under equity plans	3,160		$ 1		3,159
Issuance of common stock under equity plans (in shares)			115,000
Issuance of common stock under benefit plans	2,040		$ 1		2,039
Issuance of common stock under benefit plans (in shares)			31,000
Issuance of common stock, net of offering costs	1,139,625	$ 21,381	$ 114	$ 3	1,139,511	$ 21,378
Issuance of common stock, net of offering costs (in shares)			11,440,000	298,000
Stock-based compensation expense related to equity-classified awards	91,324				91,324
Other comprehensive loss, net of tax	(992)						(992)
Net loss	(490,874)							(490,874)
Ending Balance at Dec. 31, 2017	1,766,431		$ 997		3,947,552		(34,433)	(2,147,685)
Ending Balance (in shares) at Dec. 31, 2017			99,667,000
Exercise of common stock options, net of tax withholdings	60,743		$ 12		60,731
Exercise of common stock options, net of tax withholdings (in shares)			1,268,000
Issuance of common stock under equity plans	5,046		$ 2		5,044
Issuance of common stock under equity plans (in shares)			212,000
Issuance of common stock under benefit plans	3,170				3,170
Issuance of common stock under benefit plans (in shares)			30,000
Stock-based compensation expense related to equity-classified awards	158,642				158,642
Other comprehensive loss, net of tax	1,220						1,220
Net loss	(761,497)							(761,497)
Ending Balance at Dec. 31, 2018	$ 1,301,965		$ 1,011		4,175,139		(33,213)	(2,840,972)
Ending Balance (in shares) at Dec. 31, 2018	101,177,000		101,177,000
Exercise of common stock options, net of tax withholdings	$ 63,499		$ 15		63,484
Exercise of common stock options, net of tax withholdings (in shares)			1,374,000
Issuance of common stock under equity plans	7,909		$ 1		7,908
Issuance of common stock under equity plans (in shares)			132,000
Issuance of common stock under benefit plans	5,033		$ 1		5,032
Issuance of common stock under benefit plans (in shares)			61,000
Issuance of common stock, net of offering costs	772,477		$ 94		772,383
Issuance of common stock, net of offering costs (in shares)			9,444,000
Stock-based compensation expense related to equity-classified awards	177,230				177,230
Other comprehensive loss, net of tax	(3,305)						(3,305)
Net loss	(886,116)							(886,116)
Ending Balance at Dec. 31, 2019	$ 1,438,692		$ 1,122		$ 5,201,176		$ (36,518)	$ (3,727,088)
Ending Balance (in shares) at Dec. 31, 2019	112,188,000		112,188,000